Deformation gauges, also known as strain gauges, are resistance wire extensometers that make it possible to measure the deformation of a part by variation of the electrical resistance of the gauge (the electrical resistance increasing with the lengthening of the gauge).
These gauges are electric circuits bonded onto the parts to be studied and thus undergo a deformation similar to the deformation of the part placed under stress.
The measurement value of the gauge strongly depends on the design of the gauge but also on the linking by bonding of the gauge on the part to be studied. This linking must not vary over time (absence of viscosity) and must withstand the environment in which the gauges are placed.
In certain highly technological fields, such as for example the aeronautics field, such gauges are used to monitor the deformations and the mechanical stresses undergone by turbine engine parts during validation.
When these gauges are mounted on the rotating parts of an aeronautic turbine engine, they are subjected to important thermal stresses (of the order of 1100° C.) but also to important centrifugal stresses (the rotation speed being of the order of 20,000 rpm). Before using them for motor tests, it is thus necessary to be able to certify them and to assure that the gauges and the materials used for the linking by bonding can withstand such stresses.
However, the current approach is to dimension the strain gauges by means of empirical rules and the choice of materials to achieve the linking by bonding of the gauge is guided uniquely by the mechanical characteristics (temperature limits, etc.) given by the manufacturer.
In this context, the current method of dimensioning gauges does not make it possible to guarantee their resistance in particular conditions of operating turbine engines (i.e. with temperature stresses combined with important centrifugal stresses).
Thus, it may happen that such gauges made to rotate and under high temperature unfasten during a test. The loss of the gauge consequently implies the loss of the measurement, and occasionally multiple damage inside the turbine engine.
Furthermore, the current trend is to develop the use of strain gauges during test phases in order to recover the maximum of information, which results in a multiplication of the number of gauges in place in a turbine engine during a test. The multiplication of the number of strain gauges multiplies accordingly the risks of loss of measurement and degradation of the turbine engine, consequently implying an important risk of perturbation of test campaigns by untimely stoppages of the test and/or by the handling of the turbine engine.